1. Field of the Invention
The present invention relates to gas sensor elements, gas sensors equipped with the gas sensor element therein capable of detecting a concentration of a specific gas contained in a target gas, and a method of producing the gas sensor element.
2. Description of the Related Art
There are various types of gas sensor elements which are widely known and used.
FIG. 5 is a cross section showing a catalyst layer in such a conventional gas sensor element. FIG. 6A is a view to explain a state where a target gas is passing through a trap layer and the catalyst layer in the conventional gas sensor element 9. FIG. 6B is a view to explain a state where the target gas reaches a target gas chamber 940 in the conventional gas sensor element.
For example, as shown in FIG. 5, FIG. 6A, and FIG. 6B, one type of those conventional gas sensor element is comprised of a solid electrolyte 911, a target gas electrode 912, a reference gas electrode 913, a porous diffusion resistance layer 914, and a catalyst layer 92. The solid electrolyte 911 has a oxygen ion conductivity. The target gas electrode 912 is formed on one surface of the solid electrolyte 911. The reference gas electrode 913 is formed on the other surface of the solid electrolyte 911. As shown in FIG. 6A and FIG. 6B, the porous diffusion resistance layer 914 surrounds the target gas electrode 912. The target gas to be detected passes through the porous diffusion resistance layer 914, and reaches the target gas electrode 912. The catalyst layer 92 is formed at the outer surface of the porous diffusion resistance layer 914 through which the target gas is introduced into the inside of the gas sensor element 9. The catalyst layer 92 contains noble metal catalyst 921.
The target gas contains CO gas, NO gas, and H2 gas. Combustion of H2 gas is performed in the catalyst layer 92 formed in the gas sensor element 9. When the gas sensor element 9 has no catalyst layer, H2 gas is not burned and reaches the target gas electrode 912 in the target gas chamber 940 at high speed rather than O2 gas. This makes it for the gas sensor element 9 to output an incorrect detection signal which is away from a true detection signal regarding a concentration of a specific gas contained in the target gas because H2 gas has a light molecular weight and is capable of being rapidly passing through the inside of the porous diffusion resistance layer 914 when compared with other gases such as O2 gas.
Further, for example, there is a tendency to increase the amount of H2 gas in an exhaust gas emitted from a direct injection engine in addition when the engine starts to work because of having a different combustion mechanism.
Still further, there is a tendency to increase the amount of H2 gas in an exhaust gas emitted from CNG (compressed natural gas) engines because of having different fuel composition when compared with that of the gasoline engines. Therefore those engines have a problem where the gas sensor element outputs an incorrect detection signal which is different from a true detection signal regarding the concentration of a specific gas contained in a target gas because H2 gas passes at a high speed in the porous diffusion layer compared to other gases.
In order to solve the above conventional problem, as shown in FIG. 5, conventional techniques have proposed the gas sensor element 9 having the catalyst layer 92 formed on the outer surface of the porous diffusion resistance layer 914. The catalyst layer 92 contains platinum and palladium, in particular, contains 2 to 56 mass % of palladium in the entire noble metal catalyst 921. For example, Japanese patent laid open publication No. JP 2007-199046 discloses such a conventional gas sensor element capable of preventing an incorrect detection which is away from a true detection value.
However, this conventional gas sensor element 9 disclosed in JP 2007-199046 has the following problem. That is, this conventional gas sensor element can prevent occurrence of an incorrect detection result which is away from its true detection value of the target gas, but, there is a probability of deteriorating the noble metal catalyst 921 in the catalyst layer 92.
Therefore there is a strong demand to provide an improved gas sensor element capable of preventing outputting an incorrect detection signal which is away from a true detection signal and also preventing deterioration of the catalyst layer in the gas sensor element.